<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue@2.6.10/dist/vue.js"></script>
</head>
<body>
    <div id="app">
        <input type="text" v-model="msg" />
        <button @click="count++">累加count</button>
        doubleCount:{{doubleCount}}
    </div>
    <script>
        Vue.config.productionTip=false
        const vm=new Vue({
            el:"#app",
            data(){
                return{
                    msg:"",
                    count:2
                }
            },
            computed:{
                doubleCount(){
                    return this.count*2
                }
            },
            // watch可以监听一个data属性或者一个计算属性,就会触发watch监听的回调函数,我们可以在回调函数中做后续的事情
            watch:{
                msg(newVal,oldVal){
                    console.log("发送请求",newVal,oldVal);
                },
                doubleCount(){
                    console.log("重新计算了count");
                }
            }
        })
    </script>
</body>
</html>